Fuel injectors are commonly employed in internal combustion engines to provide precise metering of fuel for introduction into each combustion chamber. Additionally, the fuel injector atomizes the fuel during injection, breaking the fuel into a large number of very small particles, increasing the surface area of the fuel being injected, and allowing the oxidizer, typically ambient air, to more thoroughly mix with the fuel prior to combustion. The precise metering and atomization of the fuel reduces combustion emissions and increases the fuel efficiency of the engine.
Pressurized fuel is typically supplied to a fuel injector through a fuel rail or tube. The fuel injector functions as a valve that meters the pressurized fuel into an intake manifold or cylinder, where it mixes with an oxidant such as air to create the combustion mixture.
An electromagnetic fuel injector typically utilizes a solenoid assembly to supply an actuating force to open and close a fuel metering valve. Typically, the fuel metering valve is a plunger style needle valve which reciprocates between a closed position, where the needle is seated in a valve seat to prevent fuel from escaping through a metering orifice into the combustion chamber, and an open position, where the needle is lifted from the valve seat, allowing fuel to discharge through the metering orifice for introduction into the combustion chamber.
It is desirable, for emissions and performance, to minimize the size of the air entrained fuel particle during the intake cycle of internal combustion engine operation. The smaller fuel particle vaporizes more quickly to evenly distribute combustible fuel molecules with the oxygen supplied in the air. Large fuel particles may not vaporize completely within the combustion cycle, leading to the carburization and incomplete combustion of the fuel, which is inherently bad for performance, and emissions.
Conventional fuel injection strategy utilizes a pressure drop across an orifice to atomize the fuel with the energy stored in the fuel rail pressure. This is a limited source of energy for atomization. One approach for enhancing atomization is the use of a small orifice. That approach leads to manufacturing difficulty and increased risk of obstruction by contamination. That strategy also requires the precise regulation of fuel rail pressure as it has a direct impact on flow rate and spray geometry exiting the orifice. For that reason, a pressure regulator and rail pressure damper are often used in the fuel supply to the injectors.
Another fuel injection strategy is the use of a high rail pressure to increase the available atomization energy. That approach adds to the expense of the entire fuel system, including rails, pump, regulator and lines, which must all be optimized for operation at the higher pressure.
Typically, a volumetric chamber or “sac volume” exists between the discharge tip of the valve needle and the metering orifice. Upon seating of the needle on the valve seat, a volume of fuel remains within the sac and tends to drain through openings in the metering orifice after the metered fuel has already been discharged through the metering orifice, typically during low manifold pressure, high injector tip temperature operating conditions. This discharge produces rich combustion which generates unwanted exhaust emissions and reduces the fuel efficiency of the engine. Some of the fuel, however, remains in the sac which vaporizes and causes rich/lean shifts and hot start issues that are undesirable.
U.S. Pat. Nos. 4,877,187 and 4,784,322 show an electromagnetically actuated moveable metal bellows in combination with a piston and check valve, for pressurizing fuel in a fuel injector. That solution is complex and expensive. A conventional piston-type pumping operation similar to a diesel unit injector has also been attempted. A conventional diesel unit injector approach to a gasoline unit injector is not practical due to the lower viscosity and lubricating properties of gasoline fuels.
U.S. Pat. No. 7,077,379 discloses a piston pump fuel injector operated by a piezoelectric device, and having ball check valves at the inlet and outlet. A separate piezoelectric device-operated injection valve is used to meter fuel from the outlet.
U.S. Pat. No. 4,553,059 discloses a piezoelectric pump fuel injector having a piston pump operated by a piezoelectric device. The piston pump includes an o-ring seal and a Bellville washer return spring. The pumping chamber includes ball check valve at the inlet and a differential pressure-type injection nozzle at the outlet.
There is therefore presently a need to provide a fuel injector and method of injecting that reduces sac volume and permits precise control of injection volume, droplet size and spray geometry. Such an injector should minimize manufacturing costs. To the inventors' knowledge, no such injector is currently available.